Glycolysis

which, in the case of these biochemical reactions, becomes the final electron receptor in order to make CO2 and water. Basically, the reaction involved in aerobic respiration is this:

C6H12O6 + 6 O2 goes to 6 CO2 + 6 H2O + heat (this reaction is spontaneous and does not innately require energy).

There are also two important molecules made in these biochemical reactions: NADH and FADH2, which are used for the electron transport chain, which will be discussed. Flavin adenine dinucleotide, or FADH2, is a cofactor made during the Krebs cycle and contains hydrogen atoms, that give off electrons in the electron transport chain. Nicotinamide adenine dinucleotide, or NADH, is a related compound used in the electron transport chain as well. These are energy molecules like ATP, but are used in different reactions than ATP.

So, how much ATP is made in aerobic respiration after a molecule of glucose is used up? We’ll break it down soon but, if the system worked perfectly, 38 molecules of ATP are made from a molecule of glucose (2 from glycolysis, 2 from the Krebs cycle, and 34 from the electron transport chain) but because these reactions rely on membranes and because membranes are inherently leaky, the reactions are inefficient so that only about 28 to 30 molecules are actually created in these reactions.

This means that aerobic metabolism is up to 15 times more efficient than anaerobic metabolism. On the other hand, some anaerobic organisms, like those that use methane (methanogens), will use other organic molecules as their final electron receptor, yielding more ATP molecules than can be gotten from the typical anaerobic reactions that are seen in other animal organisms.

GLYCOLYSIS

This is the part of the ATP-producing process that takes place in the cytoplasm of the cell. It takes glucose (a single molecule per reaction) and turns it into 2 pyruvate molecules. This reaction uses up two ATP molecules but yields 4 ATP molecules so that the net gain is 2 ATP molecules. Figure 20 shows what the reactions in glycolysis look like:

Let’s look at how this process works from a biochemical standpoint:

• Step 1: This involves hexokinase, an enzyme that takes a “hexose” sugar (namely glucose) and phosphorylates it (adds a phosphate group) gotten from ATP. This is one of the reactions that uses ATP to drive the reaction. This leads to a molecule called glucose-6- phosphate or G6P. Remember that every molecule ending in ase, is an enzyme. A kinase is any enzyme that adds a phosphate molecule to another molecule. Atomic magnesium (Mg) is also involved to help shield the negative charges from the phosphate groups on the ATP molecule.

• Step 2: This starts with glucose-6-phosphate and uses phosphoglucose isomerase to make fructose-6-phosphate (F6P). Basically, it switches a few carbon atoms around so that the sugar is fructose instead of glucose. The six-

membered ring becomes a five-membered ring. One carbon is taken out of the ring but is not lost completely by the molecule.

• Step 3: This step takes fructose-6-phosphate and the enzyme phosphofructokinase to add another phosphate molecule, using another ATP molecule to create fructose-1,6-bisphosphate. This also uses atomic magnesium in order to shield the reaction from negative charges of the phosphate groups.

• Step 4: This is a cleavage reaction involving aldolase, which takes fructose-1,6bisphosphate and turns it into two molecules that have three carbon atoms in it:

The first is glyceraldehyde-3-phosphate (GAP) and the second is dihydroxyacetone phosphate (DHAP). The DHAP molecules need to be further acted on by triphosphate isomerase to turn it into a GAP (glyceraldehyde-3phosphate) molecule. These two steps lead to two molecules of GAP—not quite the pyruvate molecule we need to complete this reaction but it’s getting closer to that endpoint.

• Step 5: The basic thing that happens in this reaction is that the GAP molecules get acted upon by glyceraldehyde-3-phosphate dehydrogenase (GAPDH), which adds phosphate to the molecule to make 1,3-bisphosphoglycerate. This involves an NAD molecule as a cofactor as well as a phosphate molecule (which is where the second phosphate comes from). The NAD takes on a hydrogen atom to yield

NADH plus an extra hydrogen atom.

• Step 6: This takes 1,3-bisphosphoglycerate and ADP to make 3phosphoglycerate and ATP. It uses phosphoglycerate kinase, which takes the second phosphate molecule from 1,3-bisphosphoglycerate to make ATP from

ADP. Magnesium is a cofactor. Because there are two of these molecules, two

ATPs are made, making the net gain of ATP now zero.

• Step 7: This step rearranges the 3-phosphoglycerate using phosphoglycerate mutase to make 2-phosphoglycerate. It uses a mutase enzyme, which is an enzyme that takes a group from one position on a molecule to another. This is necessary to get the molecule in the pathway closer to its endpoint, which is pyruvate.

• Step 8: This step takes enolase, an enzyme, and removes a water from 2phosphoglycerate to phosphoenolpyruvate (PEP), resulting in PEP plus water.

Enolase is a “dehydrating” enzyme because of the removal of water.

• Step 9: This is the final step in the glycolysis pathway. It takes PEP (phosphoenolpyruvate) and removes the phosphate molecule. It takes the extra hydrogen molecule made earlier, ADP, and PEP to make pyruvate (which has no phosphate molecules on it) and gives it to ADP to make ATP. Because there are two of these per glucose molecule, it adds two more ATPs to the glycolysis pathway to make a net total of 2 ATP molecules in the pathway.

The final result is one glucose molecule leading to 2 pyruvate molecules and 2 ATP molecules. If this is all humans, for example, had, there would be a lot of pyruvate left over and only 2 ATP molecules made. This pyruvate molecule needs to go on in the process of carbon molecule breakdown so that CO2 can be made from the glucose molecule. Organisms that do not use oxygen will still use the pyruvate in the process of fermentation, which will soon be discussed.

Remember, too, that there are two NAD molecules leading into this process, which get reduced to make 2 NADH molecules. The addition of a hydrogen ion to the NAD molecule is called reduction, which is one-half of the “redox” reactions that go on in this process. Heat is given off in the process, making this an exothermic pathway. These NADH molecules go on later to make ATP in the oxidative phosphorylation pathway.

Now, pyruvate isn’t completely ready to enter the Krebs cycle or citric acid cycle just yet. There is a “connecting” reaction called oxidative decarboxylation. In it, pyruvate needs to be oxidized to make acetyl CoA, which actually enters the cycle. This gives off one molecule of CO2 and a molecule of NADH in the process. This requires pyruvate dehydrogenase complex (PDC)—located in the mitochondria of eukaryotic cells but in the cytoplasm of prokaryotes.